1. Field of the Invention
The present invention relates to profile libraries, and more particularly to improving the measurement accuracy using a wafer-thickness-dependent library.
2. Description of the Related Art
One example of optical metrology involves directing an incident beam at a structure, measuring the resulting diffracted beam, and analyzing the diffracted beam to determine various characteristics, such as the profile of the structure. In semiconductor manufacturing, optical metrology of periodic gratings is typically used for quality assurance.
For example, a periodic grating may be formed near an operating structure of a semiconductor chip. The periodic grating is then illuminated with an electromagnetic radiation, and the electromagnetic radiation that deflects off the periodic grating can be collected as a diffraction signal. The diffraction signal can then be analyzed to determine whether the periodic grating, and by extension whether the operating structure of the semiconductor chip, has been fabricated according to specifications.
In one conventional system, the diffraction signal collected from illuminating the periodic grating (the measured-diffraction signal) is compared to a library of simulated-diffraction signals. Each simulated-diffraction signal in the library is associated with a hypothetical profile. When a match is made between the measured-diffraction signal and one of the simulated-diffraction signals in the library, the hypothetical profile associated with the simulated-diffraction signal is presumed to represent the actual profile of the periodic grating, thus providing information about the operating structure which can be used for further processing, control etc.
Material variations can occur across a wafer, from wafer to wafer and from lot to lot. This variation, for example, can be due to CVD or spin on films not being uniform across the wafer, chamber to chamber variations and chamber drift in processing over time. Further, film properties can change across a wafer and/or wafer to wafer during a process, such as the etch process, due to the nature of using end pointing and sacrificial films to control a bottom CD. Etching of a film, for example, can change the optical properties and physical properties of a film. Such changes in material properties, film properties or other properties that occur during processing of the wafer can also cause variation in optical properties of the material, film, etc. Thus, optical metrology of a semiconductor wafer (by using a grating, for example) can produce measurement results that vary in accordance with the variations in the optical properties, rather than on the parameter intended to be measured.